The present invention relates to medical imaging cameras and more specifically to an imaging system including a mechanism for determining emission attenuation for compensating emission images for varying patient densities.
Single photon emission computed tomography (SPECT) examinations are carried out by injecting a dilution marker comprising a compound labeled with a radiopharmaceutical into the body of a patient to be examined. A radiopharmaceutical is a substance that emits photons at one or more energy levels. By choosing a compound that will accumulate in an organ to be imaged, compound concentration, and hence radiopharmaceutical concentration, can be substantially limited to an organ of interest. A radiopharmaceutical that emits photons or gamma emissions at a single known energy level is chosen.
While moving through a patient's blood stream the marker, including the radiopharmaceutical, becomes concentrated in the organ to be imaged. By measuring the intensity of the photons emitted from the organ, organ characteristics, including irregularities, can be identified.
To measure photon intensity a planar gamma camera is used. A gamma camera consists of a stand that supports a collimator, a scintillation crystal and a plurality of photomultiplier tubes (PMTs) in a single position with respect to a patient. The collimator typically includes a lead block with tiny holes therethrough which define preferred photon paths. The preferred paths are usually unidirectional and perpendicular to the length of the collimator. The collimator blocks emissions toward the crystal along non-preferred paths.
The scintillation crystal is positioned adjacent the collimator on a side opposite the patient. The crystal absorbs photons that pass through the collimator on a front surface and emits light from a back surface each time a photon is absorbed.
The PMTs positioned adjacent the crystal and on a side of the crystal opposite the collimator. Light emitted by the crystal is detected by the PMTs which in turn generate analog intensity signals.
A processor receives the PMT signals and digitally stores corresponding information as an M by N array of elements called pixels. The values of M and N are commonly 64 or 128 pixels across the two dimensions of the image. Together the array of pixel information is used by the processor to form an emission image corresponding to the specific camera position.
Most gamma camera systems generate a plurality of emission images, each taken by positioning the detector parallel to, and at an angle about, a rotation axis. The angle is incremented between views so that the plurality of images can be used together to construct pictures of transaxial slices of the body using algorithms and iterative methods that are well known to those skilled in the tomographic arts.
Unfortunately, because different materials are characterized by different attenuation coefficients, photons are attenuated to different degrees as they pass through different portions of a patient's body. For example, an inch of bone will typically attenuate a greater percentage of photons than an inch of tissue. Similarly, air filled space in a lung or sinus cavity will attenuate less photons than a comparable space filled with tissue or bone. In addition, photons passing through four inches of tissue will be attenuated to a greater degree than photons passing through one inch of tissue. Thus, if an organ emitting photons is located on one side of a patient's body, photon density on the organ side of the body will typically be greater than density on the other side.
Non-uniform attenuation about the organ causes emission image errors. For example, non-uniform attenuation causes artifacts in resulting images which can obscure images and reduce diagnostic effectiveness.
Attenuation caused by different body structures can be compensated for by generated a body attenuation map and using the attenuation map to correct emission images. An attenuation map is a map which clearly indicates attenuation characteristics of different portions of a patient's body. For example, a map for the chest area would indicate little attenuation in an air filled lung cavity, relatively greater attenuation in the chest muscle and still greater attenuation in rib and spinal bone sections.
In order to obtain an accurate attenuation map, a transmission imaging process is performed. In transmission imaging, body attenuation is directly measured by using transmission computed tomography techniques wherein a radiation source is used to project photons or the like through a patient's body. Radiation that is not attenuated is received by a scintillation crystal/detector on the opposite side of the patient. As with emission imaging, in transmission imaging, the source and detector are rotated about the patient to generate transmission images corresponding to a multiplicity of angles. The transmission images are reconstructed into the attenuation map using conventional tomography algorithms.
By collecting data corresponding to the intensity of the photon emissions and the intensity of the photon transmissions through the patient at the same gantry angles, a computer system uses the non-uniform attenuation map to correct emission images collected during emission studies.
Two different techniques have been used to obtain both transmission and emission images and are referred to herein as consecutive and simultaneous techniques. According to the consecutive technique, transmission images are generated either prior to or after generating emission images to generate an attenuation map that can be used to compensate for attenuation variations in later generated emission images.
Consecutive techniques have two main shortcomings. First, by generating transmission images prior to generating emission images, scan time required to generate all necessary images is approximately doubled. Second, when an attenuation map is used to compensate for attenuation variations in emission images, if patient position changed between the time when the transmission images were generated and the emission images were generated, the transmission and emission images will not correlate and the attenuation map will be useless for the purpose of compensating for non-uniform attenuation.
According to the simultaneous technique, both transmission and emission images are generated simultaneously. This technique is preferred because it is fast and the correlation problem associated with the consecutive data gathering techniques is eliminated.
Even though simultaneous imaging techniques increase imaging speed, imaging requires a relatively long period which can both cause patient discomfort and result in erroneous imaging data. In order to minimize patient exposure to radiation, the radiopharmaceutical attached to the compound marker and injected into a patient's bloodstream typically has relatively low photon emission levels. As a result, each emission image requires an appreciable amount of time (e.g. 40 seconds) to generate. Typically, to generate sufficient data to form useable tomographic images, at least 64 views equi-spaced about the 360.degree. surrounding a patient will be generated. With 64 views generated by a single camera, an entire imaging process can take longer than 40 minutes to complete. To increase image quality by reducing image granularity, the total number of images can be increased. If the number of images is doubled to 128, required imaging time can be nearly one and one-half hour.
Despite requiring an appreciable amount of tomography machine time which in and of itself is objectionable because it reduces patient throughput, prolonged imaging periods cause patients discomfort as a patient has to remain nearly completely still during the entire imaging procedure. During 40 minute or more procedures it is difficult if not impossible for most patients to remain completely still. When a patient moves, resulting images are distorted and blurred and once again, healthy tissue can be mistaken for irregular tissue or vice versa. For these reasons any method to speed up imaging without reducing accuracy is advantageous.
To increase imaging speed, many systems now employ two or more gamma cameras positioned around an imaging area to generate two or more emission images simultaneously. For example, referring to FIG. 1, one system includes two cameras 10,12 separated by a 90.degree. angle about a rotation axis. Here, both cameras 10, 12 are used for emission imaging and one of the two cameras is simultaneously used for transmission imaging with a transmission source (not shown) positioned opposite the camera providing photons at an energy level appreciably different than the emission energy levels. The emission/transmission camera collects both emission and transmission photons, identifies the different photon energy levels and generates transmission data simultaneously.
This solution has a number of shortcomings. First, a second emission camera appreciably increases system costs as scintillation crystals and PMT arrays are relatively expensive imaging components.
Second, with this solution, if imaging is required from angles throughout 360.degree. about the rotation axis, the cameras have to be rotated through 270.degree. and imaging time is reduced by only 25% with a total camera cost for the two cameras that is 100% greater than a single camera.
Third, referring again to FIG. 1, to reduce stray radiation within an imaging room, gamma cameras 10,12 typically include a radiation receiving boot 14,16 which extends laterally therearound. When two cameras 10,12 are oriented at 90.degree. about a rotation axis, the boots 14,16 interfere at a common edge and an area 18 adjacent thereto within an imaging area 20 cannot be imaged. If an organ to be imaged is located within area 18, imaging information cannot be obtained without moving the patient to a different section of the imaging area 20. While this may be acceptable in some cases, in other cases, because the imaging area 20 must be kept as small as possible to maintain a relatively small system size (i.e. small gantry and small detectors), many patients cannot be moved to more than a few positions within the imaging area. Thus, with cameras spaced at 90.degree. about the rotation axis, the effective imaging area 20 is substantially reduced.
Another solution to increase imaging speed is to provide three cameras and a single transmission source. Two of the cameras can be used to detect emission information and the third camera can be positioned opposite the transmission source to detect transmission information. In this case, if the three cameras are equispaced at 120.degree. intervals around the rotation axis, images throughout the 360.degree. about a rotation axis can be generated through 240.degree. of rotation and imaging time can be reduced by 33%. However, this time savings comes at the expense of three cameras, three cameras having a cost which is 200% greater than the cost of a single camera.
Another three camera configuration includes two emission cameras positioned so as to oppose each other and a third transmission camera at 90.degree. with respect to each of the emission cameras. In this case, 360.degree. imaging can be provided by rotation through 180.degree. and imaging time can be cut in half. Here, for the cost of three cameras, imaging time is reduced by 50%.
Unfortunately, while three camera systems increase speed appreciably, the costs associated with additional cameras and the transmission source cannot be justified in most cases.
Yet another solution would be to provide a line transmission source between one planar gamma camera and an imaging area wherein the line source directs gamma transmissions toward an opposed planer camera at an energy level different than the emission energy level. In this case, only two planer gamma cameras and a single line transmission source would be required thus reducing costs and still providing a fast system. With this configuration, imaging time would be reduced by as much as 50% and the two cameras would only cost twice as much as a single camera system.
Unfortunately, this configuration also has shortcomings. This configuration would cause at least one of the gamma cameras to be positioned a substantial distance away from the imaging area to accommodate the line source between the camera and the imaging area. Image quality decreases as the distance between an emission source (i.e. the radiopharmaceutical in an organ) and the camera increases. Thus, this solution would result in lower quality emission data.
Therefore, it would be advantageous to have a gamma camera system that can appreciably decrease imaging time, is relatively inexpensive and simultaneously provides both emission image data and transmission image data.